Electrophotographic photoreceptors having photosensitivity to visible light have been widely used for copying machines, printers, etc. As such electrophotographic photoreceptors, inorganic photoreceptors comprising, on an electroconductive support, with a photosensitive layer comprising selenium, zinc oxide, cadmium sulfide, and other inorganic photoconductive substances as the main ingredients have been widely used. However, these inorganic photoreceptors are not always satisfactory with respect to photosensitivity, thermal stability, waterproofness, durability and other characteristics required as electrophotographic photoreceptors for copying machines and others. For instance, selenium is inclined to be crystallized by heat or stained by such as fingerprints which easily deteriorate above characteristics. Electrophotographic photoreceptors using cadmium sulfide are inferior in waterproofness and durability and those using zinc oxide have a problem in durability. Electrophotographic photoreceptors using selenium and cadmium sulfide are also have disadvantageous restrictions in their manufacture and handling.
In order to improve such problems of inorganic photoconductive substances, various organic photoconductive substances have been experimentarily used for photosensitive layers of electrophotographic photoreceptors. Research and development in this field have actively evolved recently. For instance, an organic photoreceptor using a photosensitive layer containing poly-N-vinyl carbazole and 2, 4, 7-trinitrofluorenone is disclosed in Japanese Unexamined Patent Publication No. 50-10496. This photoreceptor is, however, not sufficient with respect to sensitivity and durability. Therefore, an electrophotographic photoreceptor of the separated function type with two layers, a photosensitive layer composing a carrier generating layer and a carrier transporting layer independently, which respectively contain a carrier generating substance and carrier transporting substance, were developed. This enables different substances to assume the carrier generating function and carrier transporting function independently. Therefore such substances that have one of the functions can be selected from wide range.
Thus, it is expected to obtain organic photoreceptors with high sensitivity and durability. Many carrier generating substances effective for the carrier generating layer of electrophotographic photoreceptors of the separate function type have been proposed. As an example of those using inorganic substances, amorphous selenium can be used as presented in the Gazette for the Japanese Unexamined Patent Publication No. 43-16198. This carrier generating layer containing the amorphous selenium is used in combination with a carrier transporting layer containing organic carrier transporting substance. However, the carrier generating layer comprising the amorphous selenium has the trouble of crystallization due to heat resulting in deterioration of the characteristics as described above. As an example using an organic substance as the carrier generating substance, there are organic dyes or pigments. For instances, those with a photosensitive layer containing bis-azo compounds represented in the Gazettes for Japanese Unexamined Patent Publication Nos. 47-37543, 55-22834, 54-79632, 56-116040, etc. have been already known.
However, though these bis-azo compounds represent relatively favorable sensitivity in the short and medium wavelength ranges, they are low in sensitivity in long wavelength range. It was difficult to use them in laser printers which use semiconductor laser beam sources and they require high reliability.
The gallium aluminum arsenide (Ga. Al. As) type light emitting element which is now widely used as semiconductor laser is more than 750 mm in oscillating wavelength. In order to obtain electrophotographic photoreceptors of high sensitivity for such long wave length light, many studies have been done. For instance, such a method was conceived as to add sensitizing agent to photosensitive materials such as Se, CdS and others with high sensitivity in the visible light range to make the wavelength longer. As described above, however, Se and CdS have not yet sufficient environmental resistance to temperature, humidity, etc. Also, a large number of organic type photoconductive materials have been known as described above; their sensitivity is limited to the visible light region below 700 nm usually and only a very small number of materials have enough sensitivity for longer wavelengths.
Among them phthalocyanine type compounds are known to have make photosensitivity than other especially in the long wavelength region. Among them, .alpha.-type titanyl phthalocyanine is presented in the Gazzette for the Japanese Unexamined Patent Publication No. 61-239248. This type titanyl phthalocyanine has the Bragg angle peaks when exposed to X-rays of CuK.alpha. 1.541.ANG. at 7.5, 12.3, 16.3, 25.3, and 28.7. However, its sensitivity is low and electric potential stability is inferior in repeated use and is susceptible to photographic fog in electrophotographic processes using reversal development. Electrification power is also low and a sufficient image density is hard to obtain.